Electromagnetic telemetry gap sub assembly with insulating collar

ABSTRACT

An insulating collar for a gap sub assembly for electromagnetic (EM) telemetry used in downhole drilling is disclosed. The gap sub assembly comprises a female member comprising a female mating section and a male member comprising a male mating section and a gap section. The male mating section is matingly received within the female mating section and electrically isolated therefrom. The insulating collar is positioned on the gap section. The collar is made up of a framework with a plurality of discrete bodies spaced about the framework and a portion of each of the discrete bodies protrudes above the framework. Either the framework or the discrete bodies are made of an electrical insulator material to electrically isolate one end of the collar from the other end of the collar. The collar therefore electrically isolates the male member from the female member and the male member, female member and insulating collar function as the “gap sub” for EM telemetry.

REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Application No. 61/727,610filed 16 Nov. 2012 and U.S. Application No. 61/767,759 filed 21 Feb.2013. For purposes of the United States, this application claims thebenefit under 35 U.S.C. §119 of U.S. Application No. 61/727,610 filed 16Nov. 2012 and U.S. Application No. 61/767,759 filed 21 Feb. 2013, bothof which are entitled ELECTROMAGNETIC TELEMETRY GAP SUB ASSEMBLY WITHINSULATING COLLAR and which are hereby incorporated herein by referencefor all purposes.

FIELD

This disclosure relates generally to gap sub assemblies andelectrically-insulating collars for gap sub assemblies. Embodimentsprovide gap sub assemblies suitable for use in measurement whiledrilling using electromagnetic telemetry and methods for fabricating gapsub assemblies.

BACKGROUND

The recovery of hydrocarbons from subterranean zones relies on theprocess of drilling wellbores. This process includes drilling equipmentsituated at the surface and a drill string extending from the surfaceequipment to the formation or subterranean zone of interest. The drillstring can extend thousands of feet or meters below the surface. Theterminal end of the drill string includes a drill bit for drilling, orextending, the wellbore. The process also relies on some sort ofdrilling fluid system, in most cases a drilling “mud”. The mud is pumpedthrough the inside of the drill string, which cools and lubricates thedrill bit and then exits out of the drill bit and carries rock cuttingsback to surface. The mud also helps control bottom hole pressure andprevents hydrocarbon influx from the formation into the wellbore andpotential blow out at the surface.

Directional drilling is the process of steering a well from vertical tointersect a target endpoint or to follow a prescribed path. At theterminal end of the drill string is a bottom hole assembly (BHA) whichmay include 1) the drill bit; 2) a steerable downhole mud motor of arotary steerable system; 3) sensors of survey equipment for loggingwhile drilling (LWD) and/or measurement while drilling (MWD) to evaluatedownhole conditions as drilling progresses; 4) apparatus for telemetryof data to the surface; and 5) other control equipment such asstabilizers or heavy weight drill collars. The BHA is conveyed into thewellbore by a string of metallic tubulars known as the drill string. MWDequipment may be used to provide downhole sensor and status informationat the surface while drilling in a near real-time mode. This informationis used by the rig crew to make decisions about controlling and steeringthe well to optimize the drilling speed and trajectory based on numerousfactors, including lease boundaries, existing wells, formationproperties, hydrocarbon size and location. These decisions can includemaking intentional deviations from the planned wellbore path asnecessary, based on the information gathered from the downhole sensorsduring the drilling process. In its ability to obtain real time data,MWD allows for a relatively more economical and efficient drillingoperation.

Various telemetry methods may be used to send data from MWD or LWDsensors back to the surface. Such telemetry methods include, but are notlimited to, the use of hardwired drill pipe, acoustic telemetry, use offibre optic cable, mud pulse (MP) telemetry and electromagnetic (EM)telemetry.

EM telemetry involves the generation of electromagnetic waves at thewellbore which travel through the earth's surrounding formations and aredetected at the surface.

Advantages of EM telemetry relative to MP telemetry, include generallyfaster baud rates, increased reliability due to no moving downholeparts, high resistance to lost circulating material (LCM) use, andsuitability for air/underbalanced drilling. An EM system can transmitdata without a continuous fluid column; hence it is useful when there isno mud flowing. This is advantageous when the drill crew is adding a newsection of drill pipe as the EM signal can transmit the directionalsurvey while the drill crew is adding the new pipe.

Disadvantages of EM telemetry include lower depth capability,incompatibility with some formations (for example, high salt formationsand formations of high resistivity contrast), and some market resistancedue to acceptance of older established methods. Also, as the EMtransmission is strongly attenuated over long distances through theearth formations, it requires a relatively large amount of power so thatthe signals are detected at surface. Higher frequency signals attenuatefaster than low frequency signals.

A BHA metallic tubular is generally used as the dipole antennae for anEM telemetry tool by dividing the drill string into two conductivesections by an insulating joint or connector which is known in the artas a “gap sub”. One important design aspect of an EM telemetry system isthe gap sub. The gap sub must meet electrical isolation requirements aswell as withstanding the mechanical loading induced during drilling andthe high differential pressures that occur between the center andexterior of the drill pipe. These mechanical loads are typically quitehigh and most drill string components are made from high strength,ductile metal alloys in order to handle the loading without failure. Asmost high dielectric materials typically used in gap sub assemblies areeither significantly lower strength than metal alloys or highly brittle,the mechanical strength of the gap sub becomes a significant designhurdle. The gap sub tends to be a weaker link in the drill string.

Directional drilling is generally started by drilling a vertical sectionof wellbore. At some point, the drill is operated so that the wellboredeviates from the vertical forming a curve or ‘dogleg’. The trajectoryof the wellbore may change rapidly as a curve is formed in the wellbore.Direction changes that occur more rapidly than planned or desired cancause problems. For example, the casing may not fit easily through atoo-tightly curved section of the wellbore (sometimes called amicro-dogleg section). Repeated abrasion by the drill string at thedogleg can result in worn spots in which the BHA may become lodged.Excessive doglegs can also increase the overall friction of the drillstring, resulting in increased potential for damage of the BHA. Passingaround a tight dogleg can cause special problems for a gap sub includingthe potential for damage and excessive wear of the dielectric isincreased. The reduced mechanical strength of a gap sub can cause thegap to act as a flex collar which can cause excessive stress in the gapsub when undergoing bending. Such stress can cause dielectric materialin the gap to chip out, crack or buckle due to compressive loading, fromwear in the borehole, or from impact with the borehole.

SUMMARY

This invention has a number of aspects. One aspect providesconstructions for gap subs. Another aspect provides methods forfabricating gap subs. Another aspect provides gap subs having extendedgaps. Another aspect provides components for gap subs. Another aspectprovides gap subs having electrically-insulating gaps with electricalconductors extending across the gaps. Another aspect provideselectrically-insulating collars for gap subs. There is synergy amongdifferent ones of these aspects. However, the aspects also haveindependent application.

One aspect provides an insulating collar for a gap sub assembly. Thecollar has a pair of longitudinal ends spaced apart from each other anda bore therethrough. The collar comprises a framework and a plurality ofdiscrete bodies spaced about the framework. A portion of each of theplurality of discrete bodies protrudes above a surface of the framework.The framework and the plurality of discrete bodies extend between thepair of longitudinal ends of the collar and either the framework or theplurality of discrete bodies comprises an electrical insulator materialso as to electrically isolate one of the pair of longitudinal ends ofthe collar from the other of the pair of longitudinal ends of thecollar.

The framework may comprise a metal or metal alloy. The plurality ofdiscrete bodies may be spheres.

The framework may comprise one or more than one ring with opposed sidefaces. The framework may comprise a pair of end rings and some or all ofthe plurality of discrete bodies are positioned between the end rings.The framework may further comprise one or more than one internal ringpositioned between the pair of end rings. At least some of the pluralityof discrete bodies are positioned between each of the end rings and theinternal ring. The end rings may be thicker than the internal ring.

Each of the pair of end rings may comprise an outer side face and anopposed inner side face with the inner side faces facing each other,each of the inner side faces including a plurality of spaced inner sideface end ring surface depressions thereon. Each inner side face end ringsurface depression is configured to receive a portion of one of theplurality of discrete bodies therein. The outer side faces of the pairof end rings may include a plurality of spaced outer side face end ringsurface depressions thereon. Each outer side face end ring surfacedepression is configured to receive a portion of one of the plurality ofdiscrete bodies therein. The framework may further comprise one or morethan one internal ring positioned between the pair of end rings. Theinternal ring may comprise two opposed side faces with one of theopposed side faces facing the inner side face of one of the pair of endrings and the other of the opposed side faces facing the inner side faceof the other of the pair of end rings, each of the opposed side facesincluding a plurality of spaced internal ring surface depressionsthereon. Each internal ring surface depression is configured to receivea portion of one of the plurality of discrete bodies therein. Theinternal ring surface depressions of one of the opposed side faces maybe offset from the internal ring surface depressions of the other of theopposed side faces. Alternatively, the internal ring surface depressionsof one of the opposed side faces may align with the internal ringsurface depressions of the other of the opposed side faces.

The framework may comprise a helical spring and at least some of theplurality of discrete bodies are positioned between inner side faces ofthe helical spring.

The framework may further comprise a dielectric material between theplurality of discrete bodies.

The framework may comprise a sleeve with a plurality of holestherethrough and each of the plurality of holes receives at least aportion of one of the plurality of discrete bodies therethrough.

According to a second aspect of the present disclosure, there isprovided a gap sub assembly. The gap sub assembly comprises:

-   -   (a) a female member having a female mating section;    -   (b) a male member having a male mating section and a gap        section, the male mating section being inserted into the female        mating section whereby the male and female mating sections        overlap;    -   (c) an electrical isolator component located between the        overlapping male and female mating sections such that the male        and female members are mechanically coupled together but        electrically isolated from each other at their mating sections;    -   (d) an insulating collar according to the first aspect of the        present disclosure positioned on the gap section thereby        electrically isolating the male member from the female member.

The gap section may be configured to interact with at least part of theprotruding portion of the plurality of discrete bodies of the insulatingcollar to impede rotation of the insulating collar relative to the gapsection. The gap section may comprise a plurality of longitudinallyextending grooves on an external surface thereof and at least part ofthe protruding portion of the plurality of discrete bodies is receivedin one of the plurality of longitudinally extending grooves.

The male member may further comprise a shoulder section including a maleannular shoulder. The insulating collar may be positioned between themale annular shoulder and an end of the female mating section defining afemale annular shoulder. At least one of the male annular shoulder orthe female annular shoulder may comprise a plurality of spaced shouldersurface depressions thereon. Each shoulder surface depression isconfigured to receive a portion of one of the plurality of discretebodies therein.

Another aspect provides a gap sub assembly comprising: a first endcomprising a first coupling and a second end comprising a secondcoupling. The first and second ends are attached to and electricallyinsulated from one another. A reduced-diameter section extends betweenand connects the first and second ends. A collar extendscircumferentially around and along the reduced-diameter section. Thecollar comprises a plurality of metal rings, the plurality of metalrings are axially spaced apart from one another and radially spaced fromthe reduced-diameter section by electrically-insulating bodies disposedbetween adjacent ones of the plurality of rings. A dielectric materialfills voids between the metal rings.

Another aspect provides a method for making a gap sub. The methodcomprises: placing a collar around a tubular gap portion; coupling thegap portion to at least one other part to yield an assembly wherein thecollar is located between first and second shoulders; axiallycompressing the collar; and filling spaces in the collar with adielectric material.

Another aspect provides a gap sub comprising a male part and a femalepart having a female mating section configured to receive a matingsection of the male part. The mating section of the male part comprisesa first plurality of grooves extending in a first direction on a surfaceof the mating section and a second plurality of grooves extending in asecond direction that is non-parallel to the first direction, the matingsection of the female part comprises a first plurality of groovesextending in a first direction on a surface of the mating section and asecond plurality of grooves extending in a second direction that isnon-parallel to the first direction.

Another aspect provides a gap sub comprising a male part comprising abore having a first inner diameter, a normal section having a firstouter diameter, a gap region having a second outer diameter less thanthe first outer diameter, and a male mating section coupled to a femalepart comprising a female mating section and a bore. The female matingsection is configured to receive the male mating section. Anelectrically-insulating collar surrounds the gap region of the malepart. The gap region is at least 1 meter long.

Further aspects of the invention and features of a wide range ofnon-limiting embodiments of the invention are described below and/orillustrated in the drawings.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings illustrate non-limiting embodiments of theinvention.

FIG. 1 is a schematic illustration showing a drilling site in whichelectromagnetic (EM) telemetry is being used for measurement whiledrilling in which embodiments of the invention can be employed.

FIG. 2 is side view of a gap sub assembly according to a firstembodiment.

FIG. 3 is a cross sectional partial view of the gap sub assembly of FIG.2.

FIG. 4A is a perspective view and FIG. 4B is a side view of a malemember of the gap sub assembly of FIG. 2.

FIG. 5 is a perspective view of an insulating collar of the gap subassembly of FIG. 2.

FIG. 6 is a perspective view of an internal ring of the insulatingcollar of FIG. 5.

FIG. 7 is a perspective view of an end ring of the insulating collar ofFIG. 5.

FIGS. 8A, 8B and 8C are side views of the end ring, internal ring andthe other end ring respectively of the insulating collar of FIG. 5.

FIG. 9 is a face view of an internal ring of the insulating collar ofFIG. 5 showing ceramic spheres seated in surface depressions on opposedside faces of the internal ring.

FIGS. 10A, 10B and 10C are side views of an end ring, internal ring andthe other end ring respectively according an alternative embodiment ofthe insulating collar.

FIG. 11 is a side view of an internal ring according to an alternativeembodiment of the insulating collar.

FIG. 12 is a side view of a portion of an insulating collar according toan alternative embodiment.

FIG. 13 is a cross sectional cut view of an insulating collar accordingto an alternative embodiment.

FIG. 14 is a cross sectional partial view of a gap sub assemblyaccording to a second embodiment.

FIGS. 15A, 15B, and 15C are a perspective view of an insulating collar,a perspective partial view of a female member, and a perspective partialview of a male member respectively of the gap sub assembly of FIG. 14.

FIG. 16 is a perspective view of an internal ring of an insulatingcollar according to an example embodiment.

FIGS. 16A and 16B are front and back views of the internal ring of FIG.16.

FIG. 17 is a cross sectional view of a pinned connection between a maleand a female member according to an example embodiment.

FIG. 18 is a cross sectional view of a connection between a male and afemale member according to an example embodiment.

FIGS. 19 and 20 are perspective view of the male and female members,respectively, of the connection in FIG. 18.

FIG. 21 is a cross section view of a connection between a male and afemale member with a compression collar.

DETAILED DESCRIPTION

The embodiments described herein generally relate to gap sub assembliesfor electromagnetic (EM) telemetry in downhole drilling. The gap subassemblies include a collar in a gap section. The gap section iselectrically insulating overall. The collar may be provided by one ormore members that extend circumferentially around the gap sub and aresupported by a plurality of discrete bodies. In some embodiments thecircumferential members comprise rings. In a non-limiting exampleembodiment the rings are metal rings and the discrete bodies compriseceramic spheres. The rings and discrete bodies may be embedded in anelectrically-insulating material. The rings may be shaped to providerecesses to receive the discrete bodies.

The collar may be generally described as including a framework with aplurality of discrete bodies spaced within the framework. In someembodiments a portion of each of the discrete bodies protrudes radiallyoutwardly past the framework. Either or both of the framework and thediscrete bodies are made of an electrical insulator material therebyelectrically isolating one end of the collar from the other end of thecollar.

The collar is supported between two parts of the gap sub assembly. Insome embodiments the gap sub assembly comprises a female membercomprising a female mating section and a male member comprising a malemating section and a gap section. The male mating section is matinglyreceived within the female mating section and electrically isolatedtherefrom. The insulating collar is positioned on the gap section.

The collar therefore electrically isolates the male member from thefemale member. The male member, female member and insulating collarfunction as the “gap sub” for EM telemetry. The male member and femalemember may each comprise a suitable coupling (e.g. an API standardthreaded coupling) for coupling the gap sub to uphole and downhole partsof the drill string.

FIG. 1 is a schematic representation of a drill site in which EMtelemetry is being applied to transmit data to the surface. Gap subassemblies according to embodiments of the present invention may beemployed in transmitting EM telemetry signals. Downhole drillingequipment including a derrick 1 with a rig floor 2 and draw works 3facilitate rotation of drill pipe 6 in the ground 5. The drill pipe 6 isenclosed in casing 8 which is fixed in position by casing cement 9.Drilling fluid 10 is pumped down drill pipe 6 and through anelectrically isolating gap sub assembly 100 to drill bit 7. The drillingfluid returns to the surface by way of annular space 11 and passesthrough a blow out preventer (BOP) 4 positioned above the groundsurface.

The gap sub assembly 100 may be positioned, for example, at the top ofthe BHA, with the BHA and the drill pipe 6 each forming part of a dipoleantenna structure. Ends of gap sub assembly 100 are electricallyisolated from one another. Gap sub assembly 100 effectively provides aninsulating break, known as a gap, between the bottom of the drill stringwith the BHA and the larger top portion of the drill string thatincludes the rest of the drill pipe 6 up to the surface.

A very low frequency alternating electrical current 14 is generated byan EM carrier frequency generator 13 and driven across the gap subassembly 100. The low frequency AC voltage is controlled in atimed/coded sequence to energize the earth and create an electricalfield 15. Communication cables 17 transmit the measurable voltagedifferential between the top of the drill string and various surfacegrounding rods 16 located about the drill site to a signal receiver box18. The grounding rods 16 may be randomly located on site with someattention to site operations and safety. A receiver box communicationcable 19 transmits the data received to a rig display 12 to providemeasurement while drilling information to the rig operator.

Some embodiments provide a gap sub construction in which a framework iscompressed between uphole and downhole shoulders. The framework maycomprise metal parts but is electrically insulating overall. Theframework may be filled with a suitable dielectric material. In suchembodiments the framework can stiffen the gap sub against bending forcesand can protect the dielectric material against damage from contact withmaterial in the wellbore.

In some embodiments the framework comprises a plurality of metal ringsthat are spaced apart from one another and from otherelectrically-conductive parts of the gap sub by electrically-insulatingbodies. The electrically insulating bodies comprise ceramic spheres insome embodiments.

FIGS. 2 and 3 illustrate an example gap sub assembly 100 in accordancewith an example embodiment of the invention. Gap sub assembly 100includes a male member 20 mated with a female member 30 and aninsulating collar 40 positioned on the male member 20 between a firstshoulder 27 on the male member and a second shoulder 37 on the femalemember. When the gap sub assembly 100 is positioned in the drill pipe 6as shown FIG. 1, the female member 30 may be uphole and the male member20 may be downhole although this orientation is not mandatory.

As shown in FIGS. 4A and 4B, male member 20 comprises an electricallyconductive body 28 with a bore therethrough. Body 28 may be circular incross-section. Body 28 has a shoulder section 21, a middle gap section22 and a mating section 23. Shoulder section 21 has a diameter greaterthan the diameters of gap section 22 and mating section 23, and formspart of the external surface of the gap sub assembly 100 shown in FIG.2. Shoulder section 21 includes an annular shoulder 27 adjacent to gapsection 22.

Mating section 23 is tapered and has an external diameter that graduallydecreases such that the external diameter of mating section 23 in thearea adjacent gap section 22 is greater than the external diameter ofmating section 23 at its end furthest from gap section 22.

Female member 30 comprises an electrically conductive body 32 with abore therethrough. Body 32 of female member 30 may be circular in crosssection. Body 32 has a mating section 31 and a non-mating section. Theinternal surface of mating section 31 has a taper that corresponds tothe taper of male mating section 23. The internal diameter of each partof female mating section 31 is greater than the external diameter of thecorresponding part of male mating section 23 so that female matingsection 31 fits over the male mating section 23 in the assembled gap subassembly 100 as shown in FIG. 3.

Male and female mating sections 23, 31 are dimensioned such that thereis a small radial gap 25 between the external surface of male matingsection 23 and the internal surface of female mating section 31 when themale and female members 20, 30 are mated together. A high dielectric,non-conductive material can be injected, inserted, placed or filled,etc. into radial gap 25. This material may be introduced into gap 25,for example in any manner known in the art.

In alternative embodiments, the male and female mating sections may notbe tapered. Additionally, or alternatively, other structures, forexample, but not limited to grooves, threads or rings (not shown) may beincluded on the internal surface of the female mating section 31 and/orthe external surface of the male mating section 23 to facilitate matingof the male and female members 20, 30.

FIG. 3 shows a male member 20 and female member 30 in matingrelationship. Collar 40 is positioned on the gap section 22 between anannular female shoulder 37 on one end of the female mating section 31and male annular shoulder 27. In some embodiments, collar 40 iscompressed between shoulders 27 and 37. In some embodiments, collar 40is compressed with a pressure of between 500 psi and 8000 psi. Collar 40may be rigid under compression such that the interaction between collar40 and shoulders 27 and 37 stiffens gap sub assembly 100 againstbending. This construction tends to prevent or reduce flexure of the gapsection 22 by transmitting mechanical loads resulting from flexing ofgap section 22 into shoulders 27, 37.

FIGS. 5 to 9 show an example insulating collar 40 comprising a pluralityof internal rings 41 positioned between two end rings 42. A plurality ofdiscrete bodies, which in the embodiment shown in FIGS. 5 to 9 arespheres 45, are seated between adjacent rings 41, 42. In one embodiment,rings 41, 42 are made of a metal or metal alloy, for example, but notlimited to, copper, copper alloys (e.g. beryllium copper), aluminium orstainless steel. In such embodiments spheres 45 are made of anelectrical insulator material, for example, but not limited to, ceramic,plastic, plastic coated metals, composite or carbides. In an alternativeembodiment, the rings 41, 42 are made of an electrical insulatormaterial, for example, but not limited to plastic and the spheres 45 aremade of a metal or metal alloy. In other alternative embodiments, bothrings 41 and 42 and spheres 45 are made of electrically insulatingmaterial(s).

Spheres 45 or other discrete bodies may support rings 41 and 42 withtheir internal faces spaced apart from male member 20. Thus, even ifrings 41, 42 are made of materials that are electrically conducting,rings 41, 42 do not provide a direct electrically-conducting path to thematerial of male member 20.

Internal rings 41 have two opposed side faces 44 extending between aninternal face 46 and an opposed external face 47. End rings 42 have aninner side face 48 and an opposed outer side face 49 spaced between aninternal face 50 and an external face 51. In the embodiment shown, theend ring internal and external faces 50, 51 are thicker than theinternal and external faces 46, 47 of internal rings 41.

FIG. 16 illustrates a ring 41B according to an improved alternativedesign. Ring 41B is similar to rings 41 except that it is tapered inthickness such that outer parts of ring 41B close to external face 47are thicker than inner parts of ring 41 b closer to internal face 46. Insome embodiments ring 41B tapers to an edge at which side faces 44 meet.In such embodiments internal face 46 may be very narrow.

When the internal rings 41 are made of metal or metal alloy, it may bebeneficial for the internal ring internal and external faces 46, 47 tobe thin so as to provide minimal electrically conductive material withinthe non-conductive gap of the gap sub assembly 100. A greater thicknessto the end ring internal and external faces 50, 51 may providestructural stability to the collar 40.

In alternative embodiments (not shown) the internal ring internal andexternal faces 46, 47 may be the same thickness as the end ring internaland external faces 50, 51, or the internal ring internal and externalfaces 46, 47 may be thicker than the end ring internal and externalfaces 50, 51 or the rings 41, 42 may be of varying size, shape, andplacement for various structural requirements.

In some embodiments, rings 41 and 42 trap spheres 45 or other discretebodies against male member 20. This is accomplished in some embodimentsby making side faces 44 of rings 41 beveled. In some embodiment sidefaces 44 have pockets for receiving spheres 45 or other bodies.

In the embodiments illustrated in FIGS. 16A and 16B, side faces 44 ofthe internal rings 41 have a plurality of surface depressions or dimples43 spaced around their surfaces. Dimples 43 on one side face 44A of eachinternal ring 41 are offset with the dimples 43 on the opposed side face44B. Offsetting of dimples 43 on opposed side faces 44A and 44B ofinternal rings 41 allows for thinner internal rings 41 as the dimples 43are offset rather than back to back. As discussed above, the use ofthinner internal rings 41 reduces the amount of electrically conductivematerial within the non-conductive gap of the gap sub assembly 100 whenthe internal rings 41 are made of metal or metal alloy. Furthermore morespheres 45 can be included in the collar 40 when the internal rings 41are thinner. This may increase the wear resistance of collar 40 as willbe discussed in more detail below.

The inner side face 48 of each of the end rings 42 also has a pluralityof dimples 43 spaced around the surface thereof. The outer side face 49may be smooth so that it can butt against the male or female shoulder27, 37. It is not necessary for there to be dimples 43 in outer sideface 49.

Collar 40 may be assembled on the gap section 22 before mating the maleand female members 20, 30 together. One of end rings 42 is placed overgap section 22 and positioned with its outer side face 49 adjacent tomale shoulder 27. Internal rings 41 are then stacked onto the gapsection 22 followed by the other end ring 42 with its inner side face 48facing the side face 44 of the adjacent internal ring 41.

Rings 41, 42 are positioned such that the dimples 43 of adjacentlyfacing internal ring side faces 44 are aligned and the dimples 43 of theend ring inner side faces 48 and the adjacently facing internal ringside face 44 are aligned. Spheres 45 are positioned between the rings41, 42 and sit in the aligned dimples 43. The profile of the dimples 43correspond to the curved profiles of spheres 45, thereby securing eachsphere 45 between the side faces 44, 48 in the assembled collar 40.

Alternatively, the stacked rings 41, 42 and spheres 45 may be assembledto form collar 40 before positioning the collar 40 onto gap section 22.

The outer surface of male member 20 may include recesses such asdimples, holes or grooves that receive spheres 45. For example, gapsection 22 may have a plurality of longitudinally extending grooves 24spaced around the circumference of the external surface of gap section22. The number of grooves 24 is dictated by the design of the collar 40as will be discussed in detail below. The geometry of the grooves 24(depth, placement, profile, length, etc.) is a function of the geometryof the collar 40 and gap section 22. The sides of spheres 45 facingtoward gap section 22 may be received in grooves 24.

Collar 40 (or alternative collars 140, 240 discussed below) may bepositioned on gap section 22 such that each of spheres 45 sits in one oflongitudinal grooves 24 of gap section 22. In the embodiments shown inFIGS. 4A and 4B, there are thirty two grooves 24 spaced around thecircumference of the gap section 22. This allows for spheres 45 in eachof the offset layers of the collar 40 shown in FIG. 5 to be received inone of grooves 24. In alternative embodiments (not shown), the number ofgrooves 24 may vary. This number of grooves 24 provided in a specificembodiment may depend on the number of spheres 45 in each layer and theoffset arrangement of the collar layers. For example, a collar made upof the rings 41, 42 of FIG. 10 may have sixteen spheres 45 in eachlayer, however the layers are not offset, therefore only sixteen grooves24 need to be present on the gap section to receive each sphere 45.Positioning of the spheres 45 in the longitudinal grooves 24 lockscollar 40 (or 140, 240) in place. This beneficially prevents rotation ortorsional movement of the collar 40, 140, 240 and thereby may increasethe torsional strength of gap section 22.

Dimples 43 may be uniformly spaced around rings 41. Grooves 24 may beuniformly spaced around the circumference of gap section 22.

The spacing of the dimples 43 around the side faces 44 of the internalrings 41 and the inner side face 48 of the end rings 42 is such thatthere are gaps between the spheres 45 seated in the dimples 43.

In the embodiments shown in FIGS. 5 to 9 rings 41 and 42 have sixteendimples 43 uniformly spaced around each of the internal ring side faces44 and each of the end ring inner side faces 48. Sixteen spheres 45 aretherefore seated between a pair of adjacent rings 41, 42, which make upone layer of the collar 40. The spheres 45 of each layer have an angularspacing of Y degrees.

In the exemplary embodiment shown in FIG. 9, there are sixteen spheres45 and Y is 22.5 degrees. As a result of offsetting of the dimples 45 ofopposed side faces 44 of each of the internal rings 41, the spheres oftwo adjacent layers are also angularly offset. The angular offset ofspheres 45 in adjacent layers is X degrees. In the exemplary embodimentshown in FIG. 9, X is one half the angle of the radial spacing of thespheres 45 in the adjacent layer, therefore X is 11.25 degrees. Thespheres 45 of each layer are therefore located in alternating fashionwhen viewed longitudinally along the collar 40, with alignment of thespheres 45 of layers 1, 3, 5 etc and alignment of the spheres 45 oflayers 2, 4, 6 etc.

In an alternative embodiment as shown in FIGS. 14 and 15A-C, the outerside face 49 a of end rings 42 a of insulating collar 40 a includespaced dimples 43 and corresponding aligning dimples 43 are included onthe surfaces of male and female shoulders 27 a, 37 a of male and femalemembers 20 a, 30 a respectively. The dimples 43 on the male shoulder 27a align with the longitudinal grooves 24 a of the gap section 22 a.Spheres 45 are positioned between the end rings 42 a and the male andfemale shoulders 27 a, 37 a. In an alternative embodiment (not shown)only one of the end rings 42 a and one of the corresponding male orfemale shoulders 27 a, 37 a may have dimples 43 thereon for positioningof spheres 45 therein.

The dimples 43 of the outer side face 49 a of each end ring 42 a areoffset from the dimples 43 on the inner side face 48 a of that end ring42 a, so that the spheres 45 positioned between the outer side faces 49a and the male and female shoulders 27 a, 37 a are offset from thespheres 45 in adjacent layers of collar 40 a. In an alternativeembodiment (not shown) the dimples 43 on the outer side face 49 a ofeach end ring 42 a align back to back with the dimples 43 on the innerside face 48 a of that end ring 42 a.

In alternative embodiments (not shown) the number of spheres 45 in eachlayer may be more or less than sixteen depending on the size of therings 41, 42, the size of the spheres 45 and the spacing between eachsphere 45. Furthermore, the spacing of the dimples 43, and thus thespheres 45, may be random rather than uniform. Furthermore, in analternative embodiment (not shown), the radial offset X of spheres 45 ofadjacent layers of the collar 40 may be more than or less than half theradial spacing Y between the spheres 45. For example X may be one thirdof Y so that spheres of the 1^(st), 4^(th), 7^(th) layer etc. align,spheres of the 2^(nd), 5^(th), 8^(th) layer etc. align, and spheres ofthe 3^(rd), 6^(th), 9^(th) layers etc. align. Alternative embodiments(not shown) may use a different pattern of radial spacing of spheres 45.Other innovative aspects of the invention apply equally in embodimentssuch as these.

In an alternative embodiment shown in FIG. 10, the internal ring 41 ahas dimples 43 in back to back alignment on each opposed side faces 44 aof the internal ring 41 a, such that spheres 45 positioned between theinternal and end rings 41 a, 42 will be aligned rather than offset.Alignment of spheres 45 back to back may beneficially transmit stressesmore readily for specific drilling applications and may providestructural strength and stiffness to the collar, which may be importantwhen there are high stresses on the gap sub assembly, for example whenthe downhole drilling trajectory encompasses a number of curves.

As discussed above with regards to the embodiment shown in FIGS. 5 to 9,the end rings 42 of this alternative embodiment may optionally includedimples 43 on the outer side face 49, such that spheres 45 can bepositioned between the end rings 42 and the male and female shoulders27, 37. The dimples 43 of the outer side face 49 of the end rings 42 mayalign back to back or may be offset from the dimples 43 on the innerside face 48 of the end rings 42 in this alternative embodiment.

In a further alternative embodiment shown in FIG. 11, an internal ring41 b has undulating side faces 44 b and surface depressions 43 b areprovided as a result of the undulating side faces 44 b. The surfacedepressions 43 b are offset on opposed side faces 44 b of the internalring 41 b. The end rings may also be undulating (not shown) and spheres45 may be positioned between the surface depressions of the outer sideface of the end rings and the male and female shoulders 27, 37.Alternatively, the end rings may be as shown in FIGS. 8 and 10.

It is evident from the foregoing that while the embodiments shown inFIGS. 5 to 11, utilize spheres 45 and dimples 43 or surface depressions43 b with a curved profile, in alternative embodimentsdifferently-shaped discrete bodies, such as cuboids, cube, cylinder oregg shaped bodies may be used. In these alternative embodiments theprofile of the dimples 43 or surface depressions 43 b on the internalring side faces 44, 44 a, 44 b and the end ring inner side faces 48 (andoptionally the end ring outer side faces 49) may correspond with theprofile of the discrete bodies so that the discrete bodies are securelyseated between the side faces 44, 44 a, 44 b, 48, 49.

Furthermore, in alternative embodiments there may be no dimples 43 onthe ring faces 44, 41 a, 48, 49 and the discrete bodies may be securedbetween the rings 41, 41 a, 42 in some other way, for example using anadhesive or another structural feature such as a protrusion from thesurface of the rings (not shown). Other innovative aspects of theinvention apply equally in embodiments such as these.

It can be desirable to apply compressive pre-load to collar 40. Suchpreloading may be achieved in various ways.

One way to apply compressive preloading to collar 40 is to insert wedgesor the like (not shown) made of any dielectric and/or conductivematerial between one or both of the male and female shoulders 27, 37 andthe outer side face 49 of the adjacent end rings 42.

Another way to apply compressive pre-loading to collar 40 is to press orpull on male and female members 20, 30 so as to force male shoulder 27toward female shoulder 37 before mating male and female members 20, 30to one another.

Another way to apply compressive pre-loading to collar 40 is to providean electrically-insulating threaded coupling between male and femalemembers 20, 30. The threaded coupling may permit drawing male shoulder27 toward female shoulder 37 by turning male member 20 relative tofemale member 30. By way of non-limiting example, the threaded couplingmay comprise helical grooves formed on an outside diameter of matingsection 23 of male member 20 and corresponding helical grooves formed onan inside diameter of mating section 31 of female member 30. Thethreaded connection may be completed by providing electricallyinsulating members (such as electrically insulating spheres for example)that engage the grooves in the male and female members. An example ofthis construction is described elsewhere herein.

Another way to apply compressive loading to collar 40 is to provide highstrength electrically insulating rods or cords that extend across gapsection 22 (for example between rings 41, 42 and male member 20) and canbe tightened to draw shoulders 27, 37 toward one another.

Another way to apply compressive loading to collar 40 is to provide amember adjacent to shoulder 27 that has internal threads that engagecorresponding threads on the outer diameter of male member 20 at the endof gap section 22 adjacent to shoulder section 21. The member may beturned relative to male member 20 so that it advances toward shoulder 37to compress collar 40. The member may have holes passing through it tofacilitate filling both sides of the member with a suitable dielectricmaterial as discussed below. In an alternative embodiment a threadedmember is adjacent shoulder 37 and can be turned to compress collar 40against shoulder 27.

Another way to apply compressive loading to collar 40 is to provide amember adjacent to shoulder 27 or 37 that can be forced toward theopposing shoulder 37 or 27 by way of suitable cams, wedges, bolts or thelike.

Once collar 40 is positioned on the gap section 22 female member 30 canbe mated with male member 20 to form the gap sub assembly 100. Wherecollar 40 will be compressively pre-loaded then, depending on themechanism for applying the pre-loading, the preloading may be performedbefore, after or as part of mating male section 20 to female section 20.A suitable dielectric material may then be applied to fill the spacesaround collar 40.

Providing a collar 40 that is compressed can increase resistance of thegap section to bending. Essentially, collar 40 may carry forces betweenshoulders 27 and 37 thereby resisting bending. Collar 40 functions inplace of solid material that would be present in a section of drillstring lacking a gap section. A gap section which includes a collar 40may approximate the resistance to bending of an equivalent section ofdrill string. In some embodiments, the section of drill string havingcollar 40 has a Young's modulus which is at least 100%, 99%, 95%, 90%,80%, 70%, or 50% of the Young's modulus of an equivalent section ofdrill string that does not have a gap section. An equivalent section ofdrill string may comprise a section of drill string with the samematerial, outer diameter and bore diameter as gap sub assembly 100 butmade of solid metal.

In some embodiments compressive forces applied to collar 40 aretransmitted by way of a ring and the points at which forces are appliedto one side face of the ring are angularly offset relative to the pointsat which forces are applied to the opposing side face of the ring. Theseforces can therefore cause some bending of the ring which may act as astiff spring, In such embodiments, forces which attempt to bend the gapsub will attempt to further compress collar 40 along one side of the gapsub. Collar 40 can resist such further compression thereby stiffeningthe gap sub against bending. The stiffness of collar 40 may be adjustedby selecting the construction of the rings, the material of the rings,the width of the rings, the thickness of the rings, the ring geometry,and/or the number of spheres 45 or other discrete bodies spaced aroundthe rings. Stiffness may be increased by increasing the number ofspheres 45 in each layer of collar 40 (all other factors being equal).

Female member 20 may be mated to male member 30 in various ways. Forexample, the dielectric material may hold male part 20 to female part30. Projections, indentations or the like may be provided in one or bothof male member 20 and female member 30 to better engage the dielectricmaterial.

As another example, male member 20 may be pinned to female member 30using electrically insulating pins, bolts or the like. Male and femalemembers may also or in the alternative be pinned together with metalpins. The metal pins may be attached at one end to one of male member 20and female member 30 (for example by being press-fit, welded in place,or the like. The other end of the metal pins may pass through anaperture in the other member (either male member 20 or female member30). The aperture is large enough that the metal pin does not contactthe material of the other member directly. An electrically insulatingmaterial fills the space in the aperture surrounding the second end ofthe metal pin. The electrically insulating material may, for example,comprise a moldable dielectric material. In some embodiments, some pinsare attached to male member 20 and pass through apertures in femalemember 20 and some pins are attached to female member 30 and passthrough apertures in male member 20. In each case the pins areelectrically insulated from the member that they are not attached to.

In some embodiments, some or all of the pins are made of an insulatingmaterial. In some embodiments, some or all of the pins are not directlyattached to either male member 20 or female member 30, but are insertedthrough apertures in female member into a corresponding bore in malemember 20. These inserted pins may be held in place by an injecteddielectric material, an adhesive, or the force of friction.

A high dielectric, non conductive material, for example, but not limitedto, an injectable thermoplastic or epoxy or engineered resin is injectedinto the radial gap 25 between the external surface of the male matingsection 23 and the internal surface of the female mating section 31. Theinjected dielectric material sets and electrically isolates the malemating section 23 from the female mating section 31, as well aspreventing drilling fluid from filling the radial gap 25. The dielectricmaterial may additionally help to attach male member 20 to female member30.

FIG. 17 shows an example of a pinned connection between male member 20and female member 30. In this example, a pin 60A is attached to andprojects outwardly from male member 20 into an aperture 61A in femalemember 30. A dielectric material 62 fills aperture 61A around pin 60A.Also shown is a pin 60B that is attached to and projects inwardly fromfemale member 30 into an aperture 61B in male member 20. The portion ofaperture 61B around pin 60B is filled with dielectric material 62. Thedielectric material 62 may also fill the gap 25 between male member 20and female member 30.

The number of pins and their locations may be varied. Pins 60A and/or60B may be spaced apart around the circumferences of male member 20 andfemale member 30. Different pins 60A and/or 60B may be at the sameand/or different axial positions along male member 20 and female member30.

As another example, male member 20 may be held to female member 30 byproviding electrically-insulating bodies (e.g. spheres) that engagegrooves or other indentations in male member 20 and female member 30.The electrically-insulating bodies may be inserted into gap 25 throughapertures in female member 30. An example embodiment having thisconstruction is discussed below and illustrated in FIGS. 18-20. In someembodiments male member 20 has a plurality of sets of grooves in matingsection 23 and female member 30 has a corresponding plurality of sets ofgrooves in mating section 31. The grooves of different ones of the setsof grooves may be non-parallel. For example, one set of grooves mayextend circumferentially around mating section 23 and another set ofgrooves may extend longitudinally in mating section 23. Bodies receivedin the first set of grooves may assist in resisting tension forces whilebodies received in the second set of grooves may assist in resistingtorques.

The same or a different dielectric material is injected into the spacesbetween the spheres 45 in each layer of collar 40 and into the spacebetween the collar 40 and the male and female shoulders 27, 37, suchthat the spheres 45 and rings 41, 42 (and wedges when present) areimmersed in the dielectric material. The injection step may be a onephase step whereby the dielectric material is injected into the radialgap 25 and into all spaces of the collar 40 and gap section 22.Alternatively, the dielectric material may be injected in the spaces ofthe collar 40 before the male and female members 20, 30 are mated. Insome embodiments, dielectric material is injected to fill collar 40before collar 40 is positioned on gap section 22. In another embodimentthe dielectric material is injected into radial gap 25 and into thespaces between rings 41, 42 in a number of steps.

It is advantageous to provide vents (for example, radially extendinggrooves) on outer side faces 49 of end rings 42. Such vents can aid inensuring that the injected dielectric material suitably embeds end rings42. The extrusion of small amounts of dielectric material through suchvents can be used as an indication that the dielectric material isfilling collar 40.

One advantage of making collar 40 using rings 41, 42 that have a taperedcross-section or otherwise provide undercuts on side faces 44, 48, 49 isthat such rings help to retain the dielectric material in the spacesbetween adjacent rings 41, 42. When rings 41, 42 are tapered the spacesbetween the rings can be very generally trapezoidal in cross section. Awedging action between the dielectric material in such spaces and theside faces 48, 19 of the rings helps to resist tear out of thedielectric material.

The amount of dielectric material needed is reduced compared toconventional gap sub assemblies as the material need only be injected inthe spaces between the spheres 45 rather than covering the whole of thegap section 22.

In the assembled gap sub assembly 100, the spheres 45 in layers of thecollar 40 and the dielectric material creates a dielectric spaceconfined by the male and female shoulders 27, 37 and defined by thediameter of the spheres 45 and the geometry of any rings 41, 42provided.

While the embodiment shown in FIGS. 2, 3 and 5 show the insulatingcollar 40 with a plurality of internal rings 41, in an alternativeembodiment (not shown) there may be only one internal ring 41, 41 a, 41b positioned between the two end rings 42 or positioned directly betweenshoulders 27, 37.

The number of internal rings 41, 41 a, 41 b can be varied depending onthe size of the male gap section 22, which beneficially allows collar 40to be designed to fit any sized gap. An advantage of this constructionis that it permits the use of gaps that are much larger than the gaps incurrent common use. A very large gap can facilitate the use ofhigher-voltage signals for EM telemetry. This, in turn can result inimproved data communication from greater depths and/or from formationsthat are not ideal for EM telemetry. A further advantage of the use of avery large gap is that the electrical power needed for EM telemetry maybe reduced.

While the gaps of typical conventional gap subs range from less than 1inch (less than 2½ cm) to a few inches (e.g. 20 cm or so), theconstruction described herein may be applied to provide gaps that aremore than 3 feet (more than about 1 meter) or 4 feet (more than about 1⅓meters) across. In some cases the gaps may exceed 10 feet (about 3meters) across. In some embodiments gaps may be 30 feet or more (about10 meters or more across).

While constructions as described herein are well suited for making gapsubs having extended gaps, a gap sub having an extended gap may be madeusing other constructions. The inventive concept of providing a gap subhaving a gap much longer than is typical in previously-available gapsubs is independent of the specific details of construction describedabove.

In a further alternative embodiment (not shown), the insulating collar40 may have no internal rings 41, 41 a, 41 b and only the two end rings42 with discrete bodies seated between the two inner side faces 48 ofthe end rings 42 and optionally between the outer side faces 49 of theend rings and the male and female shoulders 27, 37. In a furtheralternative embodiment (not shown), there may be only one internal ring41, 41 a, 41 b with discrete bodies positioned between each of theopposed side faces 44 and the male and female shoulders 27, 37. In yet afurther alternative embodiment (not shown) there may be one internalring 41, 41 a, 41 b and one end ring 42, with the outer side face 49 ofthe end ring 42 adjacent either one of the male or female shoulders 27,37 and discrete bodies positioned between the inner side face 48 of theend ring 42 and one of the opposed side faces 44, 44 a, 44 b of theinternal ring 41, 41 a, 41 b and between the other of the opposed sidefaces 44, 44 a, 44 b of the internal ring 41, 41 a, 41 b and the otherof the male or female shoulders 27, 37. The rings may not be circularand could for example be oval, square, or slit rings. The rings may alsobe double rings. Other innovative aspects of the invention apply equallyin embodiments such as these.

Advantageously, rings 41, 42 may be made of or have their external faces47, 51 coated with or formed of a hard abrasion-resistant metal. In suchembodiments, rings 41, 42 protect the dielectric material that fills thespaces between the rings from abrasion. The material of rings 41, 42 ispreferably not so brittle that rings 41 or 42 will break under expectedoperating conditions.

As shown for example in FIG. 11, in some embodiments, rings 41, 42 mayhave undulating side faces. Even rings which do not have undulating sidefaces, may deform as a result of axial compression of collar 40 so thattheir side faces undulate to some degree. Rings may optionally bemachined to provide undulating side faces. Undulating side faces ofrings 41 and 42 can be advantageous for helping to prevent scouring ofthe dielectric material between the rings by formations encountereddownhole.

FIGS. 18-20 show a portion of a gap sub 300 according to another exampleembodiment. Gap sub 300 comprises a male part 20 and a female part 30which may be substantially as described above. A collar 40 (not shown inFIGS. 18-20) may be supported between shoulders 27, 37 Gap sub 300provides three sets of grooves 302A, 302B and 302C in the surfaces ofmating part 23 of male part 20 and three corresponding sets of grooves303A, 303B and 303C in the surface of mating part 31 of female part 30.

Grooves 302A and 303A are helical and are configured to receive spheres45. For example, spheres 45 may be fed into gap 25 where they spanbetween groove 302A and 303A through an opening 305A that may be cappedafter spheres 45 have been inserted. It can be appreciated that withspheres 45 are in place as described, twisting female part 30 withrespect to male part 20 will result in shoulder 37 moving relative toshoulder 27. Thus, collar 40 may be axially compressed between shoulders27 and 37 by such rotation.

Grooves 302B, 302C, 303B and 303C may be used to secure male part 20 inthe mated relationship relative to female part 30. Circumferentialgrooves 302B and 303B may be located so that a groove 302B is axiallyaligned with the corresponding groove 303B when collar 40 has beenpreloaded in compression to a desired degree. With grooves 302B and 303Bso aligned, spheres 45 may be introduced into space 25 such that eachsphere spans between a groove 302B and the corresponding groove 303B.The spheres 45 may be introduced, for example, by way of openings 305Bthat may be plugged after the spheres are in place.

Similarly, male piece 20 and female piece 30 may be rotated relative toone another to achieve angular alignment of each groove 302C with acorresponding one of grooves 303C. When this alignment has beenachieved, spheres may be introduced into space 25 such that each spherespans between a groove 302C and the corresponding groove 303C. Thespheres 45 may be introduced, for example, by way of openings 305C thatmay be plugged after the spheres are in place.

FIG. 21 illustrates a portion of a gap sub 400 according to a stillfurther example embodiment. Gap sub 400 comprises a male part 20 and afemale part 30 which may be substantially as described above. A collar40 is supported between shoulders 27, 37. An axially-movable compressioncollar 402 is mounted on male part 20 adjacent to collar 40. Compressioncollar 40 may be moved to apply compressive preload to collar 40.

In the illustrated embodiment, compression collar 402 has internalthreads 403A that engage threads 403B on male part 20. In thisembodiment, compression collar 402 may be advanced toward shoulder 27 byturning compression collar 402 relative to male part 20. Compressioncollar 402 may have may have holes (not shown) passing through it tofacilitate filling both sides of the member with a suitable dielectricmaterial.

FIG. 12 shows an insulating collar 140 in accordance with anotherexample embodiment of the invention. Collar 140 comprises a helicalspring 141 having two tapered outer rings 142 such that an outer sideface 143 of the helical spring 141 lies flat against the male and femaleshoulders 27, 37 of the gap sub assembly 100 and inner side faces 144 ofthe helical spring 141 are angled compared to the outer side face 143. Aplurality of spheres 45 are positioned between the inner side faces 144.In an alternative embodiment (not shown) spheres 45 may also bepositioned between the outer side faces 143 and the male and femaleshoulders 27, 37.

In the embodiment shown in FIG. 12, the discrete bodies are spheres 45,however in alternative embodiments the discrete bodies may be of adifferent geometrical shape, for example, but not limited to, cuboids,cube, cylinder or egg shaped bodies. The spheres 45 may be secured inplace as a result of being received in depressions on the surface ofinner side faces 144 (not shown) or by some alternative means such aswith an adhesive, as discussed above in connection with FIGS. 5 to 11.The surface depressions may be provided as a result of the helicalspring having undulating inner side faces as discussed with respect tothe embodiment shown in FIG. 11.

In alternative embodiments (not shown) the rings of the helical spring141 may not be circular and could for example be oval, square, or slitrings. The rings may also be double rings. Other innovative aspects ofthe invention apply equally in embodiments such as these.

The injection step is carried out to inject dielectric material in anyspaces in the collar 140 and the collar is assembled on the gap section22 either before or after the injection step as discussed above inconnection with FIGS. 5 to 11.

In one embodiment, the helical spring 141 may be made of a metal ormetal alloy for example, but not limited to, copper, copper alloys,aluminium or stainless steel and the spheres 45 are made of anelectrical insulator material, for example, but not limited to, ceramic,plastic, plastic coated metals, composite or carbides. In thisembodiment, the conductive helical spring 141 must be electricallyisolated from the male and female shoulders 27, 37 in some way, forexample, but not limited to, having a plastic coating on the outer sidefaces 143 of the helical spring 141, positioning spheres 45 between theouter side faces 143 of the helical spring 141 and the male and femaleshoulders 27, 37, positioning electrical insulator wedges (not shown) orthe like between the outer side faces 143 of the helical spring 141 andthe male and female shoulders 27, 37, injecting dielectric materialbetween the outer side faces 143 of the helical spring 141 and the maleand female shoulders 27, 37, or any combination thereof. In analternative embodiment, the helical spring 141 is made of an electricalinsulator material, for example, but not limited to plastic and thespheres 45 are made of a metal or metal alloy.

FIG. 13 shows an insulating collar 240 in accordance with anotherexample embodiment of the invention. Collar 240 comprises a cylindricalsleeve 241 including a plurality of holes 242 therethrough which areconfigured to receive a plurality of spheres 45. Spheres 45 may besecured in the holes 242 by an adhesive. Additionally, or alternatively,a dielectric material may be injected to surround the sleeve 241 and thespheres 45 and secure the sphere 45 in place and may fill any gapsbetween the sleeve and the male and female shoulders 27, 37. In theembodiment shown in FIG. 13, the discrete bodies are spheres 45, howeverin alternative embodiments the discrete bodies may be of a differentgeometrical shape, for example, but not limited to, cuboids, cube,cylinder or egg shaped bodies and the holes 242 are shaped to receivethe different shaped discrete bodies. In an alternative embodiment (notshown) the holes 242 may have a smaller cross-sectional area than thelargest cross-sectional area of the discrete bodies such that only aportion of the discrete body protrudes through the hole. In thisembodiment the widest part of the discrete body is positioned betweenthe gap section 22 and the sleeve 241, therefore the discrete bodiescannot pass through the holes 242. The discrete bodies are seated in thelongitudinal grooves 24 of the gap section 22 and the sleeve 241 locksthe bodies in place within the grooves 24.

In one embodiment, the sleeve 241 may be made of a metal or metal alloyfor example, but not limited to, copper, copper alloys, aluminium orstainless steel and the spheres 45 are made of an electrical insulatormaterial, for example, but not limited to, ceramic, plastic, plasticcoated metals, composite or carbides. In this embodiment, the conductivesleeve 241 must be electrically isolated from the male and femaleshoulders 27, 37 in some way, for example, but not limited to, having aplastic coating on outer side faces 243 of the sleeve 241, positioningspheres 45 between the outer side faces 243 of the sleeve 241 and themale and female shoulders 27, 37, positioning electrical insulatorwedges or the like between the outer side faces 243 of the sleeve 241and the male and female shoulders 27, 37, injecting dielectric materialbetween the outer side faces 243 of the sleeve 241 and the male andfemale shoulders 27, 37, or any combination thereof. In an alternativeembodiment, the sleeve 241 is made of an electrical insulator material,for example, but not limited to plastic and the spheres 45 are made of ametal or metal alloy.

In some embodiments, portions of some or all of spheres 45 projectradially outward past the external faces of rings 41, 42. In suchembodiments the projecting spheres 45 or other shaped discrete bodiestherefore act as the first contact impact zone on the external surfaceof the collar 40, 140, 240. The discrete bodies may also projectradially outward from the external surfaces of the male and femalemembers 20, 30. Side impact loading may beneficially be improved as theprojected surface of the discrete bodies typically deflect impactstresses more readily than conventional sleeves positioned over the gapsection 22 that may crack or chip. The discrete bodies may also providea higher resistance to fracture and a higher resistance to wear causedby drilling fluid, thereby increasing the resistance potential of thegap sub assembly 100 of the disclosed embodiments compared toconventional gap sub assemblies. The projecting discrete bodies mayserve as wear indicators.

In some embodiments, most of spheres 45 (or other discrete bodies) donot project radially past the external surfaces of rings 41, 42. A fewspheres 45 may be mounted so that they do project radially past theexternal surfaces of rings 41, 42. The projecting spheres or otherdiscrete bodies may serve as wear indicators. Where spheres 45 engagelongitudinal grooves 24, some spheres 45 may be made to project radiallyfarther than others by making a few of longitudinal grooves 24 shallowerthan others and/or by providing shallower portions in one or more of thelongitudinal grooves. For example, several of longitudinal grooves 24spaced apart around the circumference of male member 20 may be madeshallower than others. In a specific example embodiment, four of grooves24 angularly spaced apart by 90 degrees from one another are madeshallower than the remainder of longitudinal grooves 24.

In some embodiments some or all of discrete bodies (e.g. spheres 45) arerecessed below the outermost surfaces of rings 41 and 42. The distancemay be selected such that the discrete bodies begin to protrude when therings have been worn to the point that the gap sub has reached or isapproaching its wear limit.

In alternative embodiments (not shown) longitudinal grooves 24 are notpresent or are replaced with an alternative structural feature to lockthe collar 40, 140, 240 in place. For example, the gap section 22 mayinclude individual surface depression which correspond in shape to thediscrete bodies of the collar, or the gap section 22 may include surfaceprotrusions which secure the spheres 45 and/or the rings 41, 41 a, 41 b,42 of the collar 40 or the rings of the helical spring 141 of the collar140 and secure it in place to prevent rotation or torsional movement.The collar 40, 140, 240 may additionally or alternatively be securedinto place in the gap section 22 using adhesives or plastics.

In the embodiments described herein, the collar 40, 140, 240 comprises aframework which may comprise the rings 41, 41 a, 41 b, 42 of theembodiments of FIGS. 5 to 11, the helical spring 141 of the embodimentof FIG. 12, or the sleeve 241 of the embodiment of FIG. 13. Theframework may be made of a metal or metal alloy, for example, but notlimited to, copper, copper alloys, aluminium or stainless steel.Alternatively, or additionally the framework may be made of an insulatormaterial, such as plastic, or a plastic coated metal, or a dielectricnon-conductive material such as epoxy or thermoplastic. In someembodiments, exterior faces of rings 41, 41 a, 41 b, 42 have a hardnessof at least Rc 20, 40, 50, 55, 60, 65, 67, or 69.

The discrete bodies may be made of a metal or metal alloy, for example,but not limited to, copper, copper alloys, aluminium or stainless steel,or the discrete bodies may be made of an electrical insulator material,for example, but not limited to, ceramic, plastic, plastic coatedmetals, composite or carbides. Exemplary ceramics include, but are notlimited to, zirconium dioxide, yttria tetragonal zirconia polycrystal(YTZP), silicon carbide, or composites. In one embodiment, the discretebodies are made of an insulator material and the framework is made of ametal or metal alloy and/or an insulator material, however in analternative embodiment, the framework is made of an insulator materialand the discrete bodies are made of a metal or metal alloy, and/or aninsulator material. In such embodiments when the collar is positioned inthe gap section 22 it electrically isolates the male shoulder 27 fromthe female shoulder 37. It may be beneficial to have the discrete bodiesmade of an insulator material as the protruding portion of the discretebodies is in contact with the gap section 22 thereby furtherelectrically isolating the collar 40, 140, 240 from the gap section 22.It may also be beneficial to have at least part of the framework made ofa metal or metal alloy to increases the resistance, strength andstructural stability of the collar 40, 140, 240 compared to knowncollars made of non-conductive material such as plastic.

The geometry of the collar 40, 140, 240 may allow for determination ofdownhole wear characteristics of the gap sub assembly 100 following eachsuccessive use of the MWD downhole system as the wear rates between thediscrete bodies, and other materials of the collar 40, 140, 240 can becalculated and extrapolated. More specifically, as the surface of thediscrete bodies project above the external and internal surface of therest of the collar 40, 140, 240, the discrete bodies act as a wearindicator following each successive use of the MWD downhole system.Better understanding of downhole wear characteristics may result inbetter planning and greater confidence in the deployment of older orused tools. The downhole wear characteristics can also be used todetermine when the gap sub assembly 100 has reached the end of its life.

The collar 40, 140, 240 beneficially may provide mechanical strength,structure, stiffness and durability to the gap section 22 and restrictsbending of the gap section 22. The gap section 22 can therefore belonger than corresponding gap sections of conventional gap subassemblies. The downhole EM signal efficiency and signal reception ofthe EM signal at the surface may therefore be increased as a result ofthe larger gap section 22. Use of the insulating collar 40, 140, 240 ofthe disclosed embodiments may increase, amongst other things, theoverall bending strength, stiffness, torsion strength and toughness ofthe gap sub assembly 100. As the gap sub can be one of the weakest linksin the drill string, this results in greater longevity, reliability andconfidence of the EM tool. The collar 40 is typically able to withstandhigh temperatures as the structural components of the collar 40, 140,240 can withstand higher temperatures than injectable thermoplasticand/or epoxies of conventional collars. The collar 40, 140, 240 is easyto manufacture and assemble, thereby minimizing manufacturing andproduction costs. In some of the embodiments disclosed, the amount ofdielectric material which needs to be injected in the spaces between thediscrete bodies is reduced compared to a conventional solid dielectricsleeve, which may lead to reduced manufacturing costs, and improved lifeof the tool.

A number of variations are possible. For example, ceramic rings could beprovided in collar 40 in place of spheres 45 in some embodiments.

Another aspect provides methods for making gap subs. A method accordingto an example embodiment comprises placing a collar around a tubular gapportion and coupling the gap portion to at least one other part to yieldan assembly wherein the collar is located between first and secondshoulders. The method then axially compresses the collar and fillsspaces in the collar with a dielectric material while the collar remainsaxially compressed.

While the present invention is illustrated by description of severalembodiments and while the illustrative embodiments are described indetail, it is not the intention of the applicants to restrict or in anyway limit the scope of the appended claims to such detail. Additionaladvantages and modifications within the scope of the appended claimswill readily appear to those of skill in the art. The invention in itsbroader aspects is therefore not limited to the specific details,representative apparatus and methods, and illustrative examples shownand described.

Certain modifications, permutations, additions and sub-combinationsthereof are inventive and useful and are part of the invention. It istherefore intended that the following appended claims and claimshereafter introduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

INTERPRETATION OF TERMS

Unless the context clearly requires otherwise, throughout thedescription and the claims:

-   -   “comprise,” “comprising,” and the like are to be construed in an        inclusive sense, as opposed to an exclusive or exhaustive sense;        that is to say, in the sense of “including, but not limited to”.    -   “connected,” “coupled,” or any variant thereof, means any        connection or coupling, either direct or indirect, between two        or more elements; the coupling or connection between the        elements can be physical, logical, or a combination thereof.    -   “herein,” “above,” “below,” and words of similar import, when        used to describe this specification shall refer to this        specification as a whole and not to any particular portions of        this specification.    -   “or,” in reference to a list of two or more items, covers all of        the following interpretations of the word: any of the items in        the list, all of the items in the list, and any combination of        the items in the list.    -   the singular forms “a,” “an,” and “the” also include the meaning        of any appropriate plural forms.

Words that indicate directions such as “vertical,” “transverse,”“horizontal,” “upward,” “downward,” “forward,” “backward,” “inward,”“outward,” “vertical,” “transverse,” “left,” “right,” “front,” “back”,”“top,” “bottom,” “below,” “above,” “under,” and the like, used in thisdescription and any accompanying claims (where present) depend on thespecific orientation of the apparatus described and illustrated. Thesubject matter described herein may assume various alternativeorientations. Accordingly, these directional terms are not strictlydefined and should not be interpreted narrowly.

Where a component (e.g., an assembly, ring, body, device, drill stringcomponent, drill rig system, etc.) is referred to above, unlessotherwise indicated, reference to that component (including a referenceto a “means”) should be interpreted as including as equivalents of thatcomponent any component which performs the function of the describedcomponent (i.e., that is functionally equivalent), including componentswhich are not structurally equivalent to the disclosed structure whichperforms the function in the illustrated exemplary embodiments of theinvention.

Specific examples of systems, methods and apparatus have been describedherein for purposes of illustration. These are only examples. Thetechnology provided herein can be applied to systems other than theexample systems described above. Many alterations, modifications,additions, omissions and permutations are possible within the practiceof this invention. This invention includes variations on describedembodiments that would be apparent to the skilled addressee, includingvariations obtained by: replacing features, elements and/or acts withequivalent features, elements and/or acts; mixing and matching offeatures, elements and/or acts from different embodiments; combiningfeatures, elements and/or acts from embodiments as described herein withfeatures, elements and/or acts of other technology; and/or omittingcombining features, elements and/or acts from described embodiments.

It is therefore intended that the following appended claims and claimshereafter introduced are interpreted to include all such modifications,permutations, additions, omissions and sub-combinations as mayreasonably be inferred. The scope of the claims should not be limited bythe preferred embodiments set forth in the examples, but should be giventhe broadest interpretation consistent with the description as a whole.

What is claimed is:
 1. An insulating collar for a gap sub assembly, thecollar having a pair of longitudinal ends spaced apart from each otherand a bore therethrough, the collar comprising: (a) a framework; and (b)a plurality of discrete bodies spaced about the framework, a portion ofeach of the plurality of discrete bodies protruding above a surface ofthe framework, wherein the framework and the plurality of discretebodies extend between the pair of longitudinal ends of the collar andeither the framework or the plurality of discrete bodies comprises anelectrical insulator material so as to electrically isolate one of thepair of longitudinal ends of the collar from the other of the pair oflongitudinal ends of the collar.
 2. An insulating collar as claimed inclaim 1 wherein the framework comprises a metal or metal alloy.
 3. Aninsulating collar as claimed in claim 1 or 2 wherein the plurality ofdiscrete bodies are spheres.
 4. An insulating collar as claimed in anyone of claims 1 to 3 wherein the plurality of discrete bodies are madeof ceramic.
 5. An insulating collar as claimed in any one of claims 1 to4 wherein the framework comprises one or more than one ring with opposedside faces.
 6. An insulating collar as claimed in claim 5 wherein atleast one of the one or more than one ring with opposed side faces istapered in thickness such that the outer part, furthest from the bore,is thicker than the inner part, closest to the bore.
 7. An insulatingcollar as claimed in claim 5 or 6 wherein the framework comprises a pairof end rings and at least some of the plurality of discrete bodies arepositioned between the end rings.
 8. An insulating collar as claimed inclaim 7 wherein the framework further comprises one or more than oneinternal ring positioned between the pair of end rings, wherein at leastsome of the plurality of discrete bodies are positioned between each ofthe end rings and the internal ring.
 9. An insulating collar as claimedin claim 7 wherein each of the pair of end rings comprise an outer sideface and an opposed inner side face with the inner side faces facingeach other, each of the inner side faces including a plurality of spacedinner side face end ring surface depressions thereon, wherein each innerside face end ring surface depression is configured to receive a portionof one of the plurality of discrete bodies therein.
 10. An insulatingcollar as claimed in claim 9 wherein the outer side faces of the pair ofend rings include a plurality of spaced outer side face end ring surfacedepressions thereon, wherein each outer side face end ring surfacedepression is configured to receive a portion of one of the plurality ofdiscrete bodies therein.
 11. An insulating collar as claimed in claim 9wherein the outer side faces of the pair of end rings are flat.
 12. Aninsulating collar as claimed in any one of claims 9 to 11 wherein theframework further comprises one or more than one internal ringpositioned between the pair of end rings, wherein the internal ringcomprises two opposed side faces with one of the opposed side facesfacing the inner side face of one of the pair of end rings and the otherof the opposed side faces facing the inner side face of the other of thepair of end rings, each of the opposed side faces including a pluralityof spaced internal ring surface depressions thereon, wherein eachinternal ring surface depression is configured to receive a portion ofone of the plurality of discrete bodies therein.
 13. An insulatingcollar as claimed in claim 12 wherein the internal rings have undulatingside faces and the undulating side faces provide the internal ringsurface depressions.
 14. An insulating collar as claimed in claim 12wherein the internal ring surface depressions of one of the opposed sidefaces are offset from the internal ring surface depressions of the otherof the opposed side faces.
 15. An insulating collar as claimed in claim12 wherein the internal ring surface depressions of one of the opposedside faces align with the internal ring surface depressions of the otherof the opposed side faces.
 16. An insulating collar as claimed in anyone of claims 8 or 12 to 15 wherein the end rings are thicker than theinternal ring.
 17. An insulating collar as claimed in any one of claims1 to 4 wherein the framework comprises a helical spring and at leastsome of the plurality of discrete bodies are positioned between innerside faces of the helical spring.
 18. An insulating collar as claimed inany one of claims 5 to 17 wherein the framework further comprises adielectric material between the plurality of discrete bodies.
 19. Aninsulating collar as claimed in any one of claims 1 to 4 wherein theframework comprises a sleeve with a plurality of holes therethrough andeach of the plurality of holes receives at least a portion of one of theplurality of discrete bodies therethrough.
 20. An insulating collar asclaimed in any one of claims 1 to 19 wherein the portions of thediscrete bodies protruding above the surface of the formwork comprise amaterial that may be used as a wear indicator.
 21. A gap sub assemblycomprising: (a) a female member having a female mating section; (b) amale member having a male mating section and a gap section, the malemating section being inserted into the female mating section whereby themale and female mating sections overlap; (c) an electrical isolatorcomponent located between the overlapping male and female matingsections such that the male and female members are mechanically coupledtogether but electrically isolated from each other at their matingsections; and (d) an insulating collar as claimed in any one of claims 1to 20 positioned on the gap section thereby electrically isolating themale member from the female member.
 22. A gap sub assembly as claimed inclaim 21 wherein: the male mating section has a first tapered sectionthat gradually decreases in external diameter such that the externaldiameter of the first tapered section in an area adjacent to the gapsection is greater than the external diameter of the first taperedsection in the area furthest from the gap section; and the female matingsection has a second tapered section that corresponds to the firsttapered section.
 23. A gap sub assembly as claimed in any one of claims21 and 22 wherein the first and second tapered sections are dimensionedsuch that there is a radial gap between the first and second taperedsections.
 24. A gap sub assembly as claimed in claim 23 wherein theradial gap contains a dielectric material.
 25. A gap sub assembly asclaimed in any one of claims 21 to 24 wherein the gap section isconfigured to interact with at least part of the protruding portion ofthe plurality of discrete bodies of the insulating collar to impederotation of the insulating collar relative to the gap section.
 26. A gapsub assembly as claimed in claim 25 wherein the gap section comprises aplurality of longitudinally extending grooves on an external surfacethereof and at least part of the protruding portion of the plurality ofdiscrete bodies is received in one of the plurality of longitudinallyextending grooves.
 27. A gap sub assembly as claimed in any one ofclaims 21 to 26 wherein the male member further comprises a shouldersection including a male annular shoulder, wherein the insulating collaris positioned between the male annular shoulder and an end of the femalemating section defining a female annular shoulder.
 28. A gap subassembly as claimed in claim 27 wherein at least one of the male annularshoulder and the female annular shoulder comprises a plurality of spacedshoulder surface depressions thereon, wherein each shoulder surfacedepression is configured to receive a portion of one of the plurality ofdiscrete bodies therein.
 29. A gap sub comprising: a first endcomprising a first coupling and a second end comprising a secondcoupling, the first and second ends attached to and electricallyinsulated from one another; a reduced-diameter section extending betweenand connecting the first and second ends; and a collar extendingcircumferentially around and along the reduced-diameter section, thecollar comprising: a plurality of metal rings, the plurality of metalrings being axially spaced apart from one another and radially spacedfrom the reduced-diameter section by electrically-insulating bodiesdisposed between adjacent ones of the plurality of rings; and adielectric material filling voids between the metal rings.
 30. A gap subaccording to claim 29 wherein the electrically-insulating bodiescomprise ceramic spheres.
 31. A gap sub according to any one of claims29 and 30 wherein one or more ring of the plurality of rings has a sideface formed such that an axial spacing between the one or more ring andan adjacent one of the plurality of rings increases with distance froman exterior face of the ring.
 32. A gap sub according to any one ofclaims 29 and 30 wherein one or more ring of the plurality of rings hasa cross section formed such that the cross section becomes narrowertoward the center of the ring.
 33. A gap sub according to claim 32wherein side faces of the one or more ring is beveled.
 34. A gap subaccording to any one of claims 29 and 30 wherein the plurality of ringshave side faces formed to provide recesses and theelectrically-insulating bodies are engaged with the recesses.
 35. A gapsub according to claim 34 wherein the plurality of rings have undulatingside faces and the undulating side faces provide the recesses.
 36. A gapsub according to claim 34 wherein in at least some of the plurality ofrings the recesses on a first side face of the ring are angularly offsetfrom the recesses on a second side face of the ring opposed to the firstside face.
 37. A gap sub according to any one of claims 29 to 36comprising first and second shoulders respectively at first and secondends of the reduced-diameter portion wherein the collar is preloaded incompression to bear against the first and second collars with a preloadpressure.
 38. A gap sub according to claim 37 comprising a plurality ofwedges configured to provide the preload pressure by being insertedbetween the first shoulder and the collar and/or the second shoulder andthe collar.
 39. A gap sub according to claim 37 wherein the first andsecond couplings comprise first and second threaded couplings, andwherein the first and second threaded couplings are configured toprovide the preload pressure by drawing the male shoulder toward thefemale shoulder when the first end is turned relative to the second end.40. A gap sub according to claim 37 comprising one or more chordsextending across the collar, wherein the chords are configured toprovide the preload pressure.
 41. A gap sub according to claim 37comprising an annular member adjacent to either the first shoulder orthe second shoulder and extending circumferentially around the reduceddiameter section, wherein the annular member comprises a threadedportion that engages a corresponding threaded portion on the reduceddiameter section, and wherein the annular member is configured toprovide the preload pressure when the annular member is turned relativeto the reduced diameter section.
 42. A gap sub according to claim 41wherein there are one or more holes passing through the annular memberconfigured to facilitate filling the areas on both sides of the annularmember with a dielectric material.
 43. A gap sub according to claim 37wherein one of the first and second shoulders is coupled for axialmovement along the reduced-diameter section.
 44. A gap sub according toany one of claims 37 to 43 wherein the preload pressure is at least 500psi.
 45. A gap sub according to any one of claims 29 to 44 wherein thefirst and second couplings comprise a threaded coupling.
 46. A gap subaccording to any one of claims 29 to 44 wherein the first and secondcouplings comprise a pinned coupling.
 47. A gap sub according to claim46 wherein the pined coupling comprises a plurality of pins, each of theplurality of pins inserted into one of a plurality of apertures in thefirst end and one of a plurality of bores in the second end.
 48. A gapsub according to claim 47 wherein at least a first one of the pluralityof pins is non-conductive.
 49. A gap sub according to any one of claims47 and 48 wherein at least a second one of the plurality of pins isconductive.
 50. A gap sub according to claim 49 wherein the second oneof the plurality of pins is electrically coupled to the first end andelectrically insulated from the second end.
 51. A gap sub according toclaim 49 wherein the second one of the plurality of pins is electricallycoupled to the second end, and electrically insulated from the firstend.
 52. A gap sub according to any one of claims 29 to 44, wherein thefirst and second couplings comprise a pinned coupling, wherein thepinned coupling comprises a plurality of pins extending out of the firstend into a corresponding plurality of bores in the second end.
 53. A gapsub according to claim 52 wherein the plurality of pins are electricallyconductive, and wherein an insulating material prevents the plurality ofpins from directly touching the second end.
 54. A gap sub according toany one of claims 29 to 44 wherein: the first coupling comprises a firstplurality of grooves on the exterior surface of the reduced-diametersection; the second coupling comprises a second plurality of grooves onthe interior surface of a bore of the second end; the first end iscoupled to the second end by a plurality of coupling bodies; and each ofthe plurality of coupling bodies extends into one of the first pluralityof grooves and extends into one of the second plurality of grooves. 55.A gap sub according to claim 54 wherein: at least one of the firstplurality of grooves comprises a first longitudinal groove that extendslongitudinally along the exterior surface of the reduced-diametersection; at least one of the second plurality of grooves comprises asecond longitudinal groove that extends longitudinally along theinterior surface of the bore of the second end; and at least a first oneof the plurality of coupling bodies extends into both the firstlongitudinal groove and the second longitudinal groove to resistrelative rotational movement of the first end and the second end.
 56. Agap sub according to any one of claims 54 and 55 wherein: at least oneof the first plurality of grooves comprises a first circumferentialgroove that extends circumferentially around the exterior surface of thereduced-diameter section; at least one of the second plurality ofgrooves comprises a second circumferential groove that extendscircumferentially around the interior surface of the bore of the secondend; and at least a second one of the plurality of coupling bodiesextends into both the first circumferential groove and the secondcircumferential groove to resist relative longitudinal movement of thefirst end and the second end.
 57. A gap sub according to any one ofclaims 54 to 56 wherein: at least one of the first plurality of groovescomprises a first helical groove that extends helically around theexterior surface of the reduced-diameter section; at least one of thesecond plurality of grooves comprises a second helical groove thatextends helically around the interior surface of the bore of the secondend; and at least a third one of the plurality of coupling bodiesextends into both the first helical groove and the second helicalgroove.
 58. A gap sub according go any one of claims 54 to 57 whereinthe plurality of coupling bodies comprise spheres.
 59. A gap subaccording to any one of claims 29 to 58 comprising a third plurality ofgrooves extending longitudinally along the reduced-diameter sectionwherein the electrically-insulating bodies extend into the thirdplurality of grooves.
 60. A method for making a gap sub, the methodcomprising: placing a collar around a tubular gap portion; coupling thegap portion to at least one other part to yield an assembly wherein thecollar is located between first and second shoulders; axiallycompressing the collar; filling spaces in the collar with a dielectricmaterial.
 61. A method according to claim 60 wherein the collarcomprises a plurality of rings extending circumferentially around thegap portion.
 62. A method according to claim 61 wherein the collarcomprises a plurality of electrically-insulating bodies and the methodcomprises placing the bodies to support the rings radially spaced-apartfrom the gap portion.
 63. A method according to claim 62 wherein theelectrically-insulating bodies comprise spheres and the method comprisesengaging the spheres in corresponding recesses in side faces of therings.
 64. A method according to claim 62 comprising arranging theelectrically-insulating bodies to contact the rings at circumferentiallyspaced-apart locations.
 65. A method according to claim 64 comprisingcontacting one or more of the rings with the electrically-insulatingbodies such that the electrically-insulating bodies contact one sideface of the ring at locations that are angularly offset from locationsat which an opposing side face of the ring is contacted by theelectrically-insulating bodies.
 66. A method according to claim 64comprising contacting one or more of the rings with theelectrically-insulating bodies such that the electrically-insulatingbodies contact one side face of the ring at locations which areangularly aligned with locations at which an opposing side face of thering is contacted by the electrically-insulating bodies.
 67. A methodaccording to any one of claims 60 to 66 wherein compressing the collarcomprises moving the gap portion axially relative to the other part. 68.A method according to any one of claims 60 to 66 wherein compressing thecollar comprises inserting wedges between the collar and one or more ofthe first and second shoulders.
 69. A method according to any one ofclaims 60 to 66 wherein compressing the collar comprises advancing anannular member to compress the collar against the first shoulder.
 70. Amethod according to claim 69 wherein the annular member is in threadedengagement with the gap portion and advancing the annular membercomprises rotating the annular member relative to the gap portion.
 71. Amethod according to claim 70 comprising filling a gap between the secondshoulder and the annular member with the dielectric material aftercompressing the collar.
 72. A gap sub comprising: a male part, and afemale part having a female mating section configured to receive amating section of the male part, wherein: the mating section of the malepart comprises a first plurality of grooves extending in a firstdirection on a surface of the mating section and a second plurality ofgrooves extending in a second direction that is non-parallel to thefirst direction, the mating section of the female part comprises a firstplurality of grooves extending in a first direction on a surface of themating section and a second plurality of grooves extending in a seconddirection that is non-parallel to the first direction.
 73. A gap subcomprising: a male part comprising a bore having a first inner diameter,a normal section having a first outer diameter, a gap region having asecond outer diameter less than the first outer diameter, and a malemating section; a female part comprising a female mating section and abore having the first inner diameter; and an insulating collar; wherein:the female mating section is configured to receive the male matingsection; the male and female parts comprise a material; the insulatingcollar surrounds the gap region of the male part; and the gap region isat least 1 meter long.
 74. A gap sub as claimed in claim 73 wherein thegap region is at least 2½ meters long.
 75. A gap sub as claimed in claim74 wherein the gap region is at least 10 meters long.
 76. A gap sub asclaimed in any one of claims 73 to 75 wherein the Young's modulus of thegap region and surrounding insulating collar is at least 70% of theYoung's modulus of a comparison section, wherein the comparison sectionhas a length equal to the length of the gap region, a bore with an innerdiameter equal to the first inner diameter, and an outer diameter equalto the first outer diameter, and wherein the comparison section of drillstring comprises the material.
 77. A gap sub as claimed in claim 76wherein the Young's modulus of the gap region and the surroundinginsulating collar is at least 95% of the Young's modulus of thecomparison section.
 78. Apparatus having any new and inventive feature,combination of features, or sub-combination of features as describedherein.
 79. Methods having any new and inventive steps, acts,combination of steps and/or acts or sub-combination of steps and/or actsas described herein.